High-intensity burner for combustible gas

ABSTRACT

A high intensity burner is provided in which the injector for the gas-air mixture extends into a combustion chamber through a hole in a wall of the chamber which provides an annular space between the injector and the wall so that a portion of the air required for combustion is introduced into the chamber through the space between the injector and the hole in the wall of the chamber. The device is especially useful for combustion chambers operated with fuel gases such as propane and butane.

United States Patent Ponthoreau et al.

[451 Feb. 13,1973

HIGH-INTENSITY BURNER FOR COMBUSTIBLE GAS Inventors: Marcel Maximilien Ponthoreau; Claude Yves Marie Rozen, both of 16 Cognac, Charente, France Martell & Co., Cognac (Charente), France Filed: Jan. 6, 1971 Appl. No.: 104,417

Assignee:

US. Cl ..431/l85 Int. Cl ..F23k 5/00 Field of Search ..43l/l8l, 182, 183,185, 187,

References Cited UNITED STATES PATENTS 12/1933 McKee ..43l/l88 3,463,602 8/1969 Bitterlich et a1 ..431/188 1,885,067 10/1932 Primary ExaminerEdward G. Favors Att0mey-Greene and Durr [5 7 ABSTRACT A high intensity burner is provided in which the injector for the gas-air mixture extends into a combustion chamber through a hole in a wall of the chamber which provides an annular space between the injector and the wall so that a portion of the air required for combustion is introduced into the chamber through the space between the injector and the hole in the wall of the chamber. The device is especially useful for combustion chambers operated with fuel gases such as propane and butane.

6 Claims, 2 Drawing Figures PATENTEUFEBIBIWS 3. 716.324

ATTORNEY HIGH-INTENSITY BURNER FOR COMBUSTIBLE GAS For some time, attempts have been made to construct high-intensity burners, that is to say burners in which the amount of calories liberated by combustion per unit volume is very large. It is, moreover, more a matter of combustion chambers than of burners properly so-called; for example, the normal intensity of combustion is from 300 to 500,000 kcal/h per cubic meter of chamber; in a high-intensity burner it is not rear to encounter quantities of the order of to million kcal/h per cubic meter.

By reasons of this considerable rate of liberation of heat, the walls of the chamber are raised to very high temperatures, and for this reason radiate a large part of the heat of combustion of the gases. One thus has available sources of radiation at high temperature which it is easy to provide, for example, uniform heating of the products to be treated. These burners are for this reason much used in reheating furnaces, especially if one desires very rapid heating, the high temperature of the radiating source permitting very high fluxes of energy to be achieved.

Different solutions have been proposed with a view to constructing such high-intensity burners. They all include a method or means for the energetic mixing of fresh gases with the hot burnt gases, so as to obtain a very rapid rise of temperature of the fresh gases, and in consequence, a practically instantaneous ignition.

These processes have this in common, that they all necessitate a large expenditure of mechanical energy. In fact, the combustible mixture is generally forced at high speed through very fine orifices, or else it is agitated by injecting it at high speed tangentially along the wall, or again stabilization is ensured by recirculation of hot gases behind obstacles placed in the fluid stream. These devices create large losses of heat in the fresh gas circuit, and consequently necessitate elevated pressures of the mixture, obtained by high pressure blowers.

Now certain gases, such as propane and butane, are available at relatively high pressures in bottles and reservoirs. It is tempting to use this driving pressure to entrain the air with a sufficient velocityto obtain rapid mixing in the combustion chamber. Unfortunately, the gases available at high pressure, such as propane, also require a large volume of air to achieve complete combustion (23 vols. of air for one vol. of propane). The kinetic energy at exit is proportional to the pressure, and being given the high value of the factor of dilution, it is dispersed through a large volume and finally the available energy per cubic meter of mixture is slight, the more so because the efficiency of injectors falls when the factor of dilution rises. It can decrease from 50 percent, when the entraining and entrained volumes are substantially equal, to 10 percent, and falls still further when the ratio of dilution is 23, as for propane. Thus, one cannot hope for velocities greater than 7 meters per second, corresponding to a kinetic head of the order of 3 mm of water gauge, when the propane entrains the whole amount of air.

But the furnaces have their combustion chamber at a I pressure slightly lower than atmospheric pressure. The

present invention employs this reduced pressure to bring air into the furnace.

According to the present invention, a high intensity burner for combustible gas comprises a combustion chamber having a refractory wall, and a nozzle for delivering a gas/air mixture into the chamber, there being an opening in the wall providing an annular space around the nozzle, through which part of the air for combustion can enter the chamber from the exterior.

The invention will be more fully explained below in the course of description of a constructional example. This is shown schematically in the accompanying drawings, in which:

FIG. 1 shows the burner in axial section; and

FIG. 2 is an exploded, detail, perspective view of the injector for the burner.

The burner of the invention includes a central nozzle or injector 1 through which the gas mixture arrives. This injector is placed in the center of a combustion chamber 2 of conical shape, including at its lower part an abrupt contraction 3 providing a certain radial clearance 4 with respect to the injector 1. In this way, external air penetrates into the combustion chamber 2 through this annular space 4 under the action of the reduced pressure in the working space of the furnace, the reduced pressure being obtained by natural or mechanical draught, by known means.

In comparison with orthodox high-intensity burners, where all the air blown in passes through the injector, the device constructed according to the present invention presents numerous advantages.

1. Since the secondary air completes the combustion, it is not necessary to blow in a mixture containing all the air for combustion. For example, one can be content with a primary mixture containing 50 percent of the air for combustion, that is to say l 1.5 volumes of air for 1 volume of propane. The efficiency of the injector is thus higher (20 to 25 percent) and total pressures for the mixture of air and fuel of the order of 45 to 50 mm of water are very satisfactory. These pressures are sufficient to overcome the losses of head in the pipes, and to obtain speeds of 20 to 25 meters per sec. at the exit of the injector.

2. The injector itself is at a lower temperature. In fact, in existing burners, the injector is actually in contact with the refractory, to ensure tightness. The refractory being, from the very principle of the burner, operated at a very high temperature, the injector receives in consequence a large quantity of heat by conduction, and is carried to a high temperature. It is necessary to construct such injectors of refractory steel or of ceramic. In contrast, the injector according to the present invention, is separated from the refractory by the annular space 4, and heat is not conducted thereto by the refractory. Moreover, the relatively cool secondary air passing through this annular space 4 cools the injector, and thus lowers its temperature further. It is thus entirely possible to construct these injectors of ordinary steel, for injector temperatures of 275 were found experimentally while the combustion chamber was at 1400. This cooling effect may further be increased by providing the injector with fins arranged substantially parallel to its axis.

3. The substantial cooling of the injector by the secondary air also enables one to avoid blow-backs of flame with a weak mixture. In fact, it is known that a flame cannot propogate itself in an orifice the diameter of which is lower than a certain value called the limiting diameter." This diameter depends on the temperature of the orifice wall and the richness of the gas mixture. The hotter the wall is, the smaller this diameter becomes. For a temperature of 350 to 400, the flame returns whatever the value of the diameter, if the velocity of propagation of the flame becomes greater than the actual velocity of the mixture. As the injector remains at a temperature lower than this value, the flame cannot return in burners constructed according to the present invention, because the passages 5 provided in the central member 6 of the injector are of a diameter smaller than the limiting diameter. One can thus reduce the output of the burner to very slight values without fearing to see the flame return into the supply passages for the mixture.

Forvthis reason, when one shuts off the burner, one does not see the small explosion which generally occurs with other burners, for which the progressive decreasing of the exit velocity of the gas mixture below the value corresponding to the velocity of combustion of the gas mixture causes ignition of the mixture remaining in the passages. The safety of operation is thus increased, without it being necessary to rely on nonreturn devices for the flame, as is usual.

4. Finally, the flow of secondary air is a function of the reduction of pressure which prevails in the combustion chamber. One can thus regulate the excess of air at the burner in a simple manner by action on the dampers which are customarily placed at the bottom of the hearth, so as to obtain the smallest possible excess of air. One can thus be content with an injector giving a fixed ratio between the flow of air and the flow of gas, and this injector is evidently much simpler and easier to construct than those for which it is necessary to provide regulation of the richness of the mixture.

It is equally necessary that this secondary air should mix rapidly with the combustible mixture, to reach large intensities of combustion. This mixing should be obtained with the minimum of waste of propulsive energy, and it has been proved experimentally that the best way of operating consisting in dividing the supply of gas into a plurality of elementary streams, so that the mixture/air interface is as large as possible, and to communicate to the totality a rotary motion, the effects of which on the increase of speed of mixing are well known. The flame is stabilized by this rotation, the centrifugal force urging the fresh gases into contact with the hot wall, thus bringing them rapidly to the ignition temperature. On the other hand, the reduction in pressure created at the center of the vortex sucks in the burnt. gases, which are thus hot and which mix again with the fresh gases, thus accelerating the rise in temperature.

To obtain this rotation, the simplest way is to provide a device, known in itself, consisting of a central body or hub 6 force-fitted into the tube of injector 1. This central body has helical grooves 5 provided therein, the cross-sectional shape of the grooves has in itself little significance, and can be chosen so as to be easily machined. These helicalgrooves 5 induce the gyratory motion of the gas. The angle of the helix is determined empirically, to ensure the best possible operation. If the pitch is too great, the vortical movement is insufficient to ensure the clinging to the wall and the recirculation of the gases. Combustion is then retarded, so that flames have a tendency to emerge from the combustion chamber. If the pitch is too small, the vortical movement is too intense and prevents the secondary air from penetrating into the chamber, by a well known air curtain effect. A good compromise consists in choosing the helix angles between 20 and 30.

The supply of gas permits one to determine the total section of the passages, as a function of the kinetic energy available. Moreover, it is necessary that their orifices be spaced, at the periphery of the injector, by a distance equal to three or four times their diameter, the jets needing to be sufficiently separated from one another to leave between them a passage sufficient for the secondary air. Finally, the diameter of the passages 5 must be less than the limiting diameter for the flame. These different considerations permit the injector and its core body to be conveniently dimensioned.

Moreover, the upper part 7 of the hub or body 6 is shaped in the form of a cone, to facilitate the return currents of burnt gas, by eliminating the dead zone which would not fail to be produced if the injector were flat at its summit. The lower part 8 of the hub or body 6 may likewise be formed as a cone, to compel the fresh gases to pass close to the wall where the helical passages are, and likewise eliminate a dead zone.

The construction of injectors, according to the invention, is thus particularly simple, and lends itself well to mass production. The injectors properly so-called, contain only turned parts, which can be machined on automatic lathes. The hubs with helical passages can be easily machined by turning and grinding in long lengths, and then cut off to the dimension required, and the cutting-off operation can be carried out so as to obtain automatically the double-cone shape shown.

Thus, the arrangement provided according to the present invention, consisting in forming around the gas injector an annular space intended for the passage of the secondary air, provides the following great advantages:

Possibility of using a primary mixture richer in fuel gas, and in consequence available at a higher driving pressure, without it being necessary to have recourse to additional compressors to augment this pressure.

Cooling of the injector by this secondary air, from which arises the possibility of using less expensive materials for the injector.

Great flexibility of operation, the passages of slight cross-section, formed in the injector which is conveniently cooled, opposing the blow-back of flame.

Radical reduction of explosions when shutting-off the burners.

Possibilities of controlling the excess of air by regulating the draught in the combustion chamber.

Still another advantage is that, according to the invention, several chambers may be grouped. Thus, the combustion chamber 2 need not be of circular section, but can be, for example, hexagonal. This latter arrangement permits one, in cases where a battery of burners are placed side by side, to obtain thickness of dividing walls which are the same throughout, after the manner of honeycombs. This last arrangement ensures a higher uniformity of temperature in the middle of the refractory mass, and in consequence avoids the appearance of cracks due to differential expansion. Likewise, it permits one to provide a greater surface for the combustion chambers.

lclaim:

l. A high-intensity burner for combustible gas, comprising a combustion chamber having a refractory wall, and an injector for delivering a gas/air mixture into the chamber, said chamber having an end wall portion which contracts towards a central opening, said opening providing an annular space around the injector through which part of the air for combustion can enter the chamber from the exterior, said injector comprising a terminal cylindrical hub force-fitted in a tubular body, this hub carrying at its periphery several spaced helical passages for the gas/air mixture, which communicate to the latter a gyratory movement to intensify combustion.

2. A gas burner according to claim 1, in which said wall with the opening is at the bottom of said chamber and the injector extends vertically upwards into the chamber, said bottom wall of the combustion chamber having an abrupt contraction towards the center opening thereof.

3. A gas burner according to claim 1, in which the cross-section of each helical passage is sufficiently small to prevent flame blowing back along it.

4. A gas burner according to claim 1, in which the hub has conical upper and lower parts, to promote on the one hand recirculation of burnt gases, and one the other hand the division of the gas/air mixture.

5. A gas burner according to claim 2, in which the combustion chamber is of polygonal section, so that several chambers can be easily grouped in radiating panels of large surface area, their separating walls being of uniform thickness throughout.

6. In a process for heating a combustion chamber to high uniform temperatures through a gas fuel under pressure such as propane and butane, which requires a relatively high proportion of air for complete combustion, comprising providing a combustion chamber adapted to operate at a pressure below atmosphere temperature, injecting said fuel gas and a portion of the air required for combustion into said chamber through a plurality of helical passages spaced by a distance of 3 to 4 times their diameter and adapted to impart a gyratory motion to said mixture, said passages having their outlets within said chamber and the injecting pressure being supplied by the pressure of the fuel gas, and providing an opening for the entrance of an additional supply of air surrounding and concentric with the axis of said helical passages. 

1. A high-intensity burner for combustible gas, comprising a combustion chamber having a refractory wall, and an injector for delivering a gas/air mixture into the chamber, said chamber having an end wall portion which contracts towards a central opening, said opening providing an annular space around the injector through which part of the air for combustion can enter the chamber from the exterior, said injector comprising a terminal cylindrical hub force-fitted in a tubular body, this hub carrying at its periphery several spaced helical passages for the gas/air mixture, which communicate to the latter a gyratory movement to intensify combustion.
 1. A high-intensity burner for combustible gas, comprising a combustion chamber having a refractory wall, and an injector for delivering a gas/air mixture into the chamber, said chamber having an end wall portion which contracts towards a central opening, said opening providing an annular space around the injector through which part of the air for combustion can enter the chamber from the exterior, said injector comprising a terminal cylindrical hub force-fitted in a tubular body, this hub carrying at its periphery several spaced helical passages for the gas/air mixture, which communicate to the latter a gyratory movement to intensify combustion.
 2. A gas burner according to claim 1, in which said wall with the opening is at the bottom of said chamber and the injector extends vertically upwards into the chamber, said bottom wall of the combustion chamber having an abrupt contraction towards the center opening thereof.
 3. A gas burner according to claim 1, in which the cross-section of each helical passage is sufficiently small to prevent flame blowing back along it.
 4. A gas burner according to claim 1, in which the hub has conical upper and lower parts, to promote on the one hand recirculation of burnt gases, and one the other hand the division of the gas/air mixture.
 5. A gas burner according to claim 2, in which the combustion chamber is of polygonal section, so that several chambers can be easily grouped in radiating panels of large surface area, their separating walls being of uniform thickness throughout. 